Cells of the immune system such as T lymphocytes (also referred to as T cells) recognize and interact with specific antigens through receptors or receptor complexes which, upon recognition or an interaction with such antigens, cause activation of the cell. An example of such a receptor is the antigen-specific T lymphocyte receptor complex (TCR/CD3), a complex of eight proteins. The T cell receptor (TCR) is expressed on the surface of T lymphocytes. One component, CD3, which has an invariant structure, is responsible for intracellular signaling following occupancy of the TCR by ligand. The T lymphocyte receptor for antigen-CD3 complex (TCR/CD3) recognizes antigenic peptides that are presented to it by the proteins of the major histocompatibility complex (MHC). Complexes of MHC and peptide are expressed on the surface of antigen presenting cells and other T lymphocyte targets. Stimulation of the TCR/CD3 complex results in activation of the T lymphocyte and a consequent antigen-specific immune response. The TCR/CD3 complex plays a central role in the effector function and regulation of the immune system.
T lymphocytes require a second, co-stimulatory signal to become fully active. Without such a signal, T lymphocytes are either non-responsive to antigen binding to the TCR, or become anergic. Such a co-stimulatory signal, for example, is provided by CD28, a T lymphocyte protein, which interacts with CD80 and CD86 on antigen-producing cells. ICOS (Inducible COStimulator), another T lymphocyte protein, provides a co-stimulatory signal when bound to ICOS ligand. CTLA4 (cytotoxic T-Lymphocyte Antigen 4), also known as CD152, is a receptor expressed on the surface of helper T cells and CD4+ T cells, that downregulates T cell activity. Binding of CTLA4 to its cognate ligands, CD80 and CD86, results in reduced T cell activation and proliferation. PD-1 (Programmed Cell Death-1), also known as CD279, is currently understood to negatively regulate T Cell Receptor (TCR) signals, and to broadly negatively regulate immune responses.
The essential antigen-binding, signaling, and stimulatory functions of the TCR complex have been reduced by genetic recombination methods to a single polypeptide chain, generally referred to as a Chimeric Antigen Receptor (CAR). See, e.g., Eshhar, U.S. Pat. No. 7,741,465; Eshhar, U.S. Patent Application Publication No. 2012/0093842. T lymphocytes bearing such CARs are generally referred to as CAR-T lymphocytes. CARs are constructed specifically to stimulate T cell activation and proliferation in response to a specific antigen to which the CAR binds.